Worlds in Collision

Out of the Dust, A Planet is Born
Image Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

Dusty smears around distant stars are challenging current theories of planet formation.

Rocky planets are born from dust. As dust bunnies congregate, they form larger rocky objects called planetesimals. These giant rocks collide as they circle their star, eventually growing into large terrestrial planets like the Earth, Mars and Venus.

The Spitzer Space Telescope has detected the aftermath of large planetesimal collisions. When two rocks collide, the explosion sends dust and debris into space. As the collided objects orbit their star, the dust smears along with them, eventually creating a dust ring that encircles the star.

Think of it as the bloody trail of a planet as it hobbles away from a car accident.

George Rieke, Spitzer Space Telescope scientist and astronomy professor at the University of Arizona in Tucson, is the lead author on a paper describing the new observations. The paper will be published in the Astrophysical Journal.

"What we see with Spitzer is not even the tip of the iceberg," says Rieke. "It’s the little clump of snow on the tip of the iceberg."

This artist’s concept illustrates how planetary systems arise out of massive collisions between rocky bodies.
Credit: NASA

That’s because Spitzer can only detect a huge amount of dust. To make the amount of dust that is detectable by Spitzer, there must be many large objects orbiting around a star and colliding with each other.

Scientists can tell how close or far the dust is to the star by the temperature of the ring. Warmer dust rings are close to the star, in the same sort of region where the Earth circles the sun. Colder rings are further away, in the same sort of region where Neptune and Pluto orbit in our solar system.

The rings formed by planetesimal collisions don’t last long, maybe 500 thousand or a million years, because the dust is eventually sucked into or blown out by the star. That time scale indicates the planetesimal collisions are an on-going process for stars that have the dusty rings.

The ages of the stars indicate that these collisions occur much later in a star’s lifetime than had been thought. Observations of 266 stars show that in some cases, the dusty disc activity only decays away after 200 million years.

Scott Kenyon, a theorist from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., had predicted these violent planetesimal collisions only lasted for about 10 million years.

Other theories had come closer to the Spitzer observations, but they assumed the planet formation process was smoother and less chaotic. Dusty planet-forming discs, they predicted, should steadily fade away with age, with occasional flare-ups from collisions between leftover rocky bodies.

A massive collision between rocky, embryonic planets as big as mountain ranges.
Credit: NASA/JPL

"Some earlier theories had planets build like a vacuum cleaner would make the dust bunnies build," says Rieke. "That is, you have a large planet – the dust in the bag of the vacuum cleaner – and you keep sucking more dust bunnies in it joining them, rather than having planets that are a couple hundred miles in diameter or bigger run into each other and creating the huge destruction that has to reassemble itself into a full-sized planet."

Along with Spitzer observations, Rieke and his colleagues looked at older data from the Infrared Astronomical Satellite (IRAS) and the Infrared Space Observatory (ISO).

They looked for dusty discs around stars that ranged from 20 to 500 light years away. The stars are of various ages but have similar mass – about two to three times the mass of the sun. Seventy-one of those stars were found to harbor discs.

"We thought young stars, about one million years old, would have larger, brighter discs, and older stars from 10 to 100 million years old would have fainter ones," Rieke said. "But we found some young stars missing discs and some old stars with massive discs."

A massive disc of dusty planet-forming debris encircling the nearby star Fomalhaut. Click for large view. Credit: NASA/JPL-Caltech/K. Stapelfeldt (JPL

Kenyon says the new findings indicate violent collisions are a common part of terrestrial planet formation. While we knew such violent collisions were common in our solar system – the heavily cratered surface of the moon, for instance, indicates collisions frequently occurred – but we didn’t know if that was a common feature in the formation of other planetary systems. Since other systems do have violent collisions, says Kenyon, then Earth-like planets could be common in the galaxy.

Kenyon notes that the new observations don’t change the time it takes for terrestrial planets to form – the current estimate is that it takes 10 to 100 million years. But clearing out the debris in a solar system may take longer than originally thought.

The heavy bombardment from asteroids and comets as well as larger planetesimals probably affected the origin of life on Earth. Life may have started many times, only to be destroyed by a violent collision.

"We’re not altogether sure what conditions were necessary for life to form," says Rieke. "There’s a very large set of questions that we’re just starting to pursue, and this is a nice step along that road."

Related Web Pages

Visuals and animations
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Darwin Mission
Space Interferometry Mission